There are many applications where it is necessary to bond various components together, but where conventional bonding techniques are not practical. For example, when assembling electronic devices, it is often necessary to bond components that are relatively thin and fragile, and thus not conducive to being secured with mechanical devices such as screws, rivets, and the like. Furthermore, many components comprise internal sub-components that are fragile and that can be easily damaged. As a result, adhesive resins have become a preferred means for securing many types of components together. Unfortunately, adhesive resins must be allowed to cure to properly bond two or more components together. Curing at or below room temperature is often a long process which decreases production throughput and increases production costs.
Techniques exist for curing adhesive resins with UV light at room temperature. However, the adhesive resin must be directly and completely exposed to the UV light to achieve efficient curing. Unfortunately, because of the various shapes and configurations of components, such as electrical components, shadow problems can prevent the UV light from reaching some portions of the adhesive resin, thereby increasing the time required to cure the resin.
Curing adhesives by adding heat can reduce, often dramatically, the time required to cure. Various methods of applying heat to adhesive resin to facilitate curing are known. For example, bonding techniques utilizing induction heating techniques wherein heat is produced via eddy currents generated by magnetically-induced currents, are described in U.S. Pat. No. 3,620,875 to Guglielmo, Sr. et al. Unfortunately, the addition of heat via these methods can damage the components being bonded together.
Heating techniques utilizing microwave energy are described in U.S. Pat. No. 4,626,642 to Wang et al., and U.S. Pat. No. 5,338,611 to Lause et al. Wang et al. describes blending electrically conductive fibers, such as steel, aluminum, and graphite, with a thermosetting adhesive resin to accelerate the rate of cure when subjected to microwave energy from a non-variable frequency microwave source, such as a domestic kitchen microwave oven. Lause et al. describes placing a heat generating strip at the interface of thermoplastic substrates to be joined together and applying microwave energy. The strip comprises a fiber-free thermoplastic carrier polymer that is miscible with the polymer of the substrates to be joined. The strip also contains submicron carbon black particles therein for absorbing microwave energy to produce heat. The strip is designed to vanish by being incorporated into substrates to be joined when exposed to microwave energy.
The prior art methods of component bonding with microwave energy do not, however, address the problem of arcing or local heating that often results when components, including any sub-components therewithin, are exposed to microwave energy. Furthermore, the use of metallic devices for generating heat upon being exposed to microwave energy has heretofore generally been avoided because of the uncontrollable nature of such material in a microwave field. Exposing adhesive resins to single frequency microwave energy can decrease the time required to cure as compared with conventional heating techniques. Unfortunately, the time required to cure most adhesives with microwave energy is longer than many components, especially electronics components, can withstand without incurring some damage from localized heating or arcing.
In some applications, the use of adhesive resins to bond components together is not practical. It may be necessary that the bonded components have no foreign substance at the interface between them for various reasons. A requirement for precise alignment and close tolerances may also dictate that other methods of bonding be used. One method often utilized in the electronics industry when bonding polymeric components together is ultrasonic welding, wherein no adhesives are used. Heat is generated via the vibrations of the molecules of the various component surfaces to be bonded, thereby causing the component surfaces to fuse together. Unfortunately, by its very nature, the application of ultrasonic waves often causes the components themselves to vibrate. As such, proper alignment is often difficult to achieve. Furthermore, because many such components have somewhat fragile internals, they are susceptible to damage from vibrations induced by the ultrasonic waves.